Techniques for measuring network channel resistive loss between a power-sourcing apparatus and a powered device

ABSTRACT

A method and apparatus are provided for determining resistive power loss through a channel between Power Sourcing Equipment (PSE) and a Powered Device (PD). The method includes (1) receiving indication that a PSE signal measurement is available from the PSE or a PD signal measurement is available from the PD, (2) selecting, as an input parameter to a processing operation, at least one of the PSE signal measurement or the PD signal measurement, (3) performing the processing operation to calculate a resistance value indicative of the resistive power loss through the channel between the PSE and PD based on the input parameter, and (4) outputting the resistive power loss value as a result of carrying out the processing operation.

BACKGROUND

A typical phantom (or inline) power communications system includespower-sourcing communications equipment and a set of remotely-powerednetwork devices that connect to the power-sourcing communicationsequipment though a set of network cables. The power-sourcingcommunications equipment includes a power supply and transmit/receivecircuitry. During operation, the power supply provides power to theremotely-powered network devices through the network cables, and thetransmit/receive circuitry concurrently exchanges data with theremotely-powered network devices through the same network cables.Accordingly, the users of the remotely-powered network devices are notburdened with having to separately connect their devices to powersources (e.g., wall outlets).

Several conventional approaches exist for provisioning power toremotely-powered network devices over cables having some amount ofresistance. One conventional approach, which is hereinafter referred toas the conventional “over-provisioning approach”, involves the equipmentmanufacture designing the power-sourcing communications equipment for aworst-case scenario in which the power-sourcing communications equipmentconnects to a maximum number of remotely-powered network devices throughnetwork cables at their maximum specified lengths (e.g., 100 meters inaccordance with the IEEE 802.3af standard). Under this approach, theequipment manufacturer provisions particular characteristics of thepower-sourcing communications equipment for a maximum power draw (e.g.,maximum power supplied to each remote device and maximum power loss overeach network cable due to the network cables being at their maximumlengths). For example, the manufacturer makes sure the power supply islarge enough, that there are enough circuit board power planes or thatthe circuit board power planes and power converters are robust enough tocarry worst case current, and that the fan assembly is strong enough toprovide adequate cooling. Another conventional approach, which ishereinafter referred to as the conventional “statistical methods”approach, involves the equipment manufacturer designing thepower-sourcing communications equipment based on probable uses of theequipment in the field. For example, the manufacturer may offer twomodels of power-sourcing communications equipment, namely, a lower-endmodel which is designed for lower power demand situations, and ahigher-end model which is designed for higher power demand situation,and then rely on the customer to select the best-suited model for aparticular installation location. There are also industry standardswhich attempt to provide guidelines for manufacturing certain types ofpower-sourcing communications equipment. For example, the IEEE 802.3afstandard, or the newer IEEE 802.3-2005 standard, which is also calledthe “Power over Ethernet” (PoE) standard, defines ways to build Ethernetpower-sourcing equipment and powered terminals. In particular, the IEEE802.3-2005 standard identifies ways to deliver certain electricalfeatures (e.g., 48 volts) of D.C. power over unshielded twisted-pairwiring (e.g., Category 3, 5, 5e or 6 network cables, patch cables,patch-panels, outlets and connecting hardware) to a variety of Ethernetdevices or terminals such as IP phones, wireless LAN access points,laptop computers and Web cameras.

In the context of the IEEE 802.3-2005 PoE standard where thepower-sourcing communications equipment is called the PSE (PowerSourcing Equipment) and the remote device is called the PD (PoweredDevice), some PSEs include Time Domain Reflectometry circuitry whichdetermines the integrity of the cables, i.e., the data channels.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of particular embodiments of theinvention will be apparent from the following description, asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of various embodiments of theinvention.

FIG. 1 illustrates a communication system for use in embodiments of theinvention.

FIG. 2 illustrates a Powered Device for use in embodiments of theinvention.

FIG. 3 illustrates a method, which is an embodiment of the invention.

FIG. 4 illustrates a network device, which is an embodiment of theinvention and which also may be used in practicing other embodiments ofthe invention.

FIG. 5 illustrates a method, which is an embodiment of the invention.

DETAILED DESCRIPTION Overview

Certain embodiments of the present invention provide improved methodsfor measuring resistive power loss over cables connecting a PowerSourcing Equipment (PSE) to a Powered Device (PD) in order to enable thePSE to more precisely provide the correct amount of power to the PD.Other embodiments provide for an apparatus for performing theaforementioned methods.

In one embodiment, a method is provided for calculating resistive powerloss over a cable by taking multiple measurements at a PD at differentelectrical loads. This yields a very accurate result.

In another embodiment, a general method is provided for determiningresistive power loss over a cable. In this method, the PSE and PDnegotiate their respective capabilities and determine which of severalmethods for calculating the resistive power loss ought to be applied. Inone embodiment, the most accurate available method is performed, whilein an alternative embodiment, various factors are taken intoconsideration, including accuracy, speed, etc.

In another embodiment, an apparatus for performing the general method isprovided. In one embodiment, the device is a PD, while in an alternativeembodiment, the device is a PSE.

Description of Example Embodiments

Embodiments of the invention are directed to techniques for determiningpower demands using measured network channel resistance. Such techniquesenable accurate identification of power demands for powering remotedevices through data communications cables, and thus alleviate the needto over-provision power and other resources, or rely on statisticalmethods, as in conventional approaches.

FIG. 1 shows a communications system 20 (e.g., a VoIP system) which issuitable for use by various embodiments of the invention. The system 20includes a power source, such as a PSE 26, PDs 24(1), 24(2), . . . ,24(n) (generally PDs 24), and network connection cables 22(1), 22(2), .. . , 22(n) (generally network connection cables 22) therebetween. Eachnetwork cable 22 is a channel between the PSE 26 and a particular PD 24.

The PSE 26 and PD 24 are, in one embodiment, Ethernet devices operatingaccording to the 802.3-2005 PoE standard. In that case, the connectioncables 22 are Ethernet cables, which also carry D.C. electric current.Current is carried in a loop (over two or more wires contained withinthe cable 22) between the PSE 26, the PD 24, and back to the PSE 26again.

PSE 26 contains a power supply 30, a controller 32, and networkinterfaces 28(1), 28(2), . . . , 28(n) (generally network interfaces28). The controller 32 has a processor 42, memory 44, and, in someembodiments, local measurement circuitry 40.

PD 24(2), which is connected to PSE 26 by a network connection 22(2) isdepicted in greater detail in FIG. 2.

FIG. 2 depicts a PD 24 for use in an embodiment of the invention. The PD24 contains a network interface 38 and a controller 52. The controller52 contains a processor 62, memory 64, and local measurement circuitry60. The local measurement circuitry 60 contains logic 66 for producingat least two different test loads, current measuring circuitry 68,voltage measuring circuitry 70, and an analog to digital converter (ADC)72. The voltage measuring circuitry 70 is capable of measuring localvoltage, which is the voltage drop across the PD 24. The currentmeasuring circuitry 68 is capable of measuring local current, which isthe current at the PD 24 in the circuit connecting the PD 24 and the PSE26 (i.e., the connection cable 22 together with the PD 24 and the PSE26).

One embodiment of the invention is depicted in FIG. 3. A method 100 isshown for calculating the resistance over a network channel in thesystem 20 of FIG. 1, using the PD 24 of FIG. 2. The network channelresistance may be due in part to resistance over the network cable 22,or to connector, patch panel, or other path resistances. In this method100, the local measurement circuitry 60 of the PD 24 generates a firsttest load, using logic 66 (Step 110). This load causes current to flowacross the connection cable 22 between the PSE 26 and the PD 24. Thelocal measurement circuitry 60 then makes a first current measurement I₁using current measuring circuitry 68 and a first voltage measurement V₁using voltage measuring circuitry 70 (Step 120). The first currentmeasurement I₁ may be performed anywhere along the circuit connectingthe PSE 26 to the PD 24. The first voltage measurement V₁ is a measureof the potential drop across the PD 24. Then, the local measurementcircuitry 60 of the PD 24 generates a second test load, using logic 66(Step 130). The local measurement circuitry 60 then makes a secondcurrent measurement I₂ using current measuring circuitry 66 and a secondvoltage measurement V₂ using voltage measuring circuitry 68 (Step 140).The second current measurement I₂ may be performed anywhere along thecircuit connecting the PSE 26 to the PD 24. The second voltagemeasurement V₂ is a measure of the potential drop across the PD.

The values I₁, I₂, V₁, and V₂ are digitized by means of the ADC 72 andare stored in memory 64. The values stored in memory are then operatedon by the controller 52. The controller 52 computes the value of thefollowing formula:

$\begin{matrix}{R = \frac{V_{1} - V_{2}}{I_{2} - I_{1}}} & (1)\end{matrix}$

(Step 150). Although each of the values I₁, I₂, V₁, and V₂ were createdusing the same ADC 72, any systemic error in the ADC 72 is cancelled outby the subtractions and divisions. The subtractions eliminate anypotential offset errors, while the division eliminates any gain errors.R, thus calculated, is the resistance over the channel. The resistivepower loss for any particular load over a channel may be calculated bymultiplying the resistance R by the square of the current on the channelfor that load. The PD 24 is then able to request an exact powerrequirement from the PSE 26 by summing together the power required bythe PD 24 and the calculated resistive power loss over the channel. Thismeasurement technique is accurate to within approximately 3%.

It should be noted that all communication between the PD 24 and the PSE26 typically takes place over the network. Thus, if the network cables22 are Ethernet cables, a network signal is utilized over those lines.Preferably, a standard layer 2 protocol is utilized for thiscommunication, for example the Cisco Discovery Protocol (CDP).

Another embodiment is depicted in FIG. 4. Depicted is a network device90, having a network interface 92, memory 94, and a controller 96. Inone embodiment, the network device 90 is a PD 24. In an alternativeembodiment, the network device 90 is the PSE 26.

The controller 96 performs a method 200, as depicted in FIG. 5. First,the controller 96 receives indication (for example, according to theCDP) that measurement signals are available from either the PD 24 or thePSE 26 or both (step 210). Based on the signal availability and otherfactors, the controller 96 selects one or more signals to be inputs to aprocessing operation (step 220). The controller 96 then performs aprocessing operation to calculate the resistive power loss over theconnection cable 22 (step 230). This processing operation is typicallyperformed by first calculating the resistance R of the cable 22 and thenmultiplying that resistance R by the square of the current passingthrough the circuit.

The controller 96 typically selects signals as inputs based on theaccuracy of the methods of calculation available based on the choseninput signals. In an alternative embodiment, the controller 96 mayinstead use alternative factors, such as the speed of the method in atime-critical case. In some embodiments, the most accurate methodavailable may be the method 100 described above in connection with FIGS.2 and 3. Therefore, if the PD 24 is capable of creating multiple testloads and of performing local measurements of current and voltage asdepicted in FIGS. 2-3, then the controller 96 typically selects themeasurement signals from the PD 24 and performs the method 100 asdescribed above and as depicted in FIG. 3 in order to calculate theresistance of the cable 22. Recall that the controller 96 resides on thenetwork device 90, which may reside in either the PSE 26 or the PD 24.

But, if the PD 24 is not capable of creating multiple test loads and ofperforming local measurements of current and voltage as depicted in FIG.2, then the controller 96 determines an alternative method to use tocalculate or estimate the resistance R of the connection cable 22.

In one embodiment, if the PSE 26 is equipped with local measurementcircuitry 40 to measure current and voltage and if the PSE 26 is capableof providing power at least 2 voltages, then the controller 96 selectsthe signals from the PSE 26. In that case, the PSE 26 first provides afirst voltage, and the PSE measures voltage V₁ and current I₁. Then, thePSE 26 provides a second voltage, and the PSE measures voltage V₂ andcurrent I₂. Then, with P₁=V₁×I₁ and P₂=V₂×I₂, the following formula iscalculated:

$\begin{matrix}{R = \frac{P_{1} - P_{2}}{I_{1}^{2} - I_{2}^{2}}} & (2)\end{matrix}$

This formula provides a measurement of the resistance R of theconnection cable 22 that is accurate to approximately 5%.

It should be noted that errors may be introduced into this calculationif the resistance across the PD 24 varies with respect to the currentload. Most PDs use a DC-DC converter (to convert the ˜48V PoE voltage toa usable circuit voltage, such as 3.3V). The DC-DC converter may have anefficiency that varies with the input voltage. In that case, some of theresistance R, calculated according to formula 2, may be attributable tothe PD 24, rather than to the cable 22. In order to account for thiserror, the efficiency of the PD 24 may be pre-measured, obtaining a PD24 resistance as a function of load. The PD 24 would then communicate,together with each power measurement, the calculated resistance of thePD 24 at the given load. Such communication would occur, for example,according to the CDP, or another similar protocol. Formula 2 could thenbe modified to subtract the resistance attributable to the PD 24 at agiven load when calculating the resistance of the channel.

In one embodiment, if the PSE 26 is not equipped to provide power atmultiple voltages, but the PD 24 is equipped to output its known powerload P_(PD), then the controller 96 selects inputs from both the PSE 26and the PD 24. These inputs include the current I_(PSE) and the voltageV_(PSE) measured by the local measurement circuitry 40 of the PSE 26, aswell as the power load P_(PD) provided by the PD 24. The controller thenis able to calculate the resistance R of the connection cable 22according to the following formula:

$\begin{matrix}{R = \frac{\left( {I_{PSE} \times V_{PSE}} \right) - P_{PD}}{I_{PSE}^{2}}} & (3)\end{matrix}$

This formula provides a measurement that is accurate to about 20%.

This method is also susceptible to error if the resistance across the PD24 varies with respect to the current load. One way to correct for thisis if the PD 24 is capable of measuring its input voltage and theefficiency of the DC-DC converter is known as a function of voltage (asabove). In that case, the PD 24 could modify its communicated power loadP_(PD) by subtracting any power loss attributable to the inefficiency ofthe DC-DC converter. Alternatively, if the PD 24 is not capable ofmeasuring its input voltage, the PSE 26 can correct for the errorattributable to the DC-DC converter of the PD 24 by using an iterativeprocess. In that process, the PSE first calculates formula 3, and thencalculates the voltage of the PD 24 by using the PSE 26 power P_(PSE)and subtracting the estimated power loss over the channel (I_(PSE) ²×R)and then dividing the difference by the current I_(PSE). Once the PD 24voltage has thus been calculated, using the pre-determined efficiency ofthe DC-DC converter of the PD 24, the PSE 26 can estimate the resistanceof the DC-DC converter of the PD 24 to refine its calculation of formula3. This process may then be repeated iteratively to arrive at a preciseresult.

It should be noted that there may be errors in the ADC 72 of the PD.These errors may include offset errors and gain errors. Thus, the powerused by the PD 24 P_(PD) may be incorrect. If the PD 24 measures P_(PD)by measuring the local voltage and current, gain and offset errorswithin the ADC 72 may be detected by measuring the voltage and currentat two or more different loads (since the DC-DC converter consumes thesame total power, regardless of the input voltage or current, whencorrected for DC-DC converter efficiency).

In one embodiment, if the PSE 26 is equipped with Time DomainReflectometry (TDR) circuitry, then the controller 96 may alternativelyestimate the cable resistance by estimating the length L of theconnection cable 22 through a TDR process and estimating the incrementalper-unit-length resistance R_(I) of the connection cable 22. R_(I) mayeither be user-input, or it may be estimated according to the type ofcable (e.g., category 3, 5, 5e, or 6 cable). The resistance R may thenbe estimated by the following formula:

R=L×R _(I)  (4)

This formula provides an estimate that is accurate to about 30%.

In another embodiment, once the resistance R is calculated by any of theabove-described means, the resistance R is compared to a pre-determinedresistance value. For example, according to the proposed IEEE 802.3 at(PoE Plus) standard, the maximum allowed channel resistance is 25 ohms.If the measured resistance R is less than 25 ohms, then the channel isbetter than worst-case, and the PSE 26 may then provide less power thanwould otherwise be required according to the worst-case estimates. But,if the measured resistance R is greater than 25 ohms, then it means thatsomething is wrong with the system. For example, an inappropriate cablemay be in use (e.g., CAT-3, since PoE Plus requires CAT-5e or higher),or a cable length may be too long, or there might be a faultyconnection. In such a situation, in one embodiment, an error message issent to a user indicating that the cables should be checked. In anotherembodiment, there is a threshold resistance (at least as high as theworst-case allowed resistance of 25 ohms, but possibly higher), abovewhich the PSE 26 will refuse to provide power over the channel.

Thus, methods and apparatuses for computing the resistive power lossover a powered connection cable 22 in a communications system 20 aredescribed.

While various embodiments of the invention have been particularly shownand described, it will be understood by those skilled in the art thatvarious changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

For example, method 100 was described as utilizing two currentmeasurements and two voltage measurements. However, additionalmeasurements may be used to enhance the accuracy of the method. At eachtest load, it is possible that one of the measurements was in error.Thus, the current and voltage measurements at each test load may beperformed two times. If the values differ by only a small amount (or notat all), then the values may be averaged to provide a more robust value.If the values differ by more than a threshold amount, then additionalmeasurements may be performed to determine which of the previousmeasurements was in error (e.g., caused by a spike on the line).

1. A method for determining resistive power loss through a channelbetween Power Sourcing Equipment (PSE) and a Powered Device (PD), themethod comprising: receiving indication that a PSE signal measurement isavailable from the PSE or a PD signal measurement is available from thePD; selecting, as an input parameter to a processing operation, at leastone of the PSE signal measurement or the PD signal measurement; andperforming the processing operation to calculate a resistance valueindicative of the resistive power loss through the channel between thePSE and PD based on the input parameter.
 2. A method as in claim 1wherein receiving includes: obtaining the PD signal measurement from thePD, the PD signal measurement being based on (i) a first signal valueread at a connection interface of the PD while a first load is appliedby the PD to the channel and (ii) a second signal value read at theconnection interface of the PD while a second load is applied by the PDto the channel in place of the first load.
 3. A method as in claim 2wherein: the first signal value comprises a first voltage measurementand a first current measurement; the second signal value comprises asecond voltage measurement and a second current measurement; andperforming the processing operation comprises calculating a voltagedifference as the difference between the first and second voltagemeasurements, calculating a current difference as the difference betweenthe second and first current measurements, and dividing the voltagedifference by the current difference.
 4. A method as in claim 1 whereinselecting comprises choosing a method for calculating resistive powerloss based on the availability of the PSE signal measurement and the PDsignal measurement and picking an input parameter associated with thechosen method.
 5. A method as in claim 4 wherein selecting furtherincludes choosing a method that is most accurate among a variety ofmethods.
 6. A method as in claim 5 wherein choosing a method furtherincludes: if a PD signal measurement is available and if the PD iscapable of measuring local voltage and current for two different loadvalues, then selecting as the input parameter the PD signal measurement.7. A method as in claim 6 wherein receiving includes: obtaining the PDsignal measurement from the PD, the PD signal measurement being based on(i) a first signal value read at a connection interface of the PD whilea first load is applied by the PD to the channel and (ii) a secondsignal value read at the connection interface of the PD while a secondload is applied by the PD to the channel in place of the first load. 8.A method as in claim 7 wherein: the first signal value comprises a firstvoltage measurement and a first current measurement; the second signalvalue comprises a second voltage measurement and a second currentmeasurement; and performing the processing operation comprisessubtracting the second voltage measurement from the first voltagemeasurement and dividing the difference by the first current measurementminus the second current measurement.
 9. A method as in claim 5 whereinselecting further includes: if a PD signal measurement is not availableor if the PD is not capable of measuring local voltage and current fortwo different load values then: if the PSE is capable of measuring localvoltage and current for two different load values, then selecting as theinput parameter the PSE signal measurement; otherwise if the PSE iscapable of measuring local voltage and current for one load value andthe PD is capable of reporting its power load, then selecting as theinput parameter the PSE signal measurement and the PD signalmeasurement; otherwise selecting as the input parameter the PSE signalmeasurement.
 10. A method as in claim 9 wherein receiving includes: ifthe PD signal measurement is selected, obtaining the PD signalmeasurement from the PD, the PD signal measurement being based on aplurality of measurements taken at different loads, the plurality ofmeasurements being used to eliminate any offset or gain errors.
 11. Amethod as in claim 4 wherein selecting further includes choosing amethod that is fastest among a variety of methods.
 12. A method as inclaim 1 wherein the method further comprises: if the resistance ishigher than a pre-determined value, instructing the PSE to ceaseapplying power to the PD.
 13. An apparatus comprising; memory; a networkinterface; and a controller, the controller configured to: receiveindication that a Power Sourcing Equipment (PSE) signal measurement isavailable from a PSE or a Powered Device (PD) signal measurement isavailable from a PD; select, as an input parameter to a processingoperation, at least one of the PSE signal measurement or the PD signalmeasurement; and perform the processing operation to calculate aresistance value indicative of a resistive power loss through thechannel between the PSE and PD based on the input parameter.
 14. Anapparatus as in claim 13 wherein the controller is configured such thatwhen it receives indication that the PSE signal measurement or PD signalmeasurement is available it is configured to: obtain the PD signalmeasurement from the PD, the PD signal measurement being based on (i) afirst signal value read at a connection interface of the PD while afirst load is applied by the PD to the channel and (ii) a second signalvalue read at the connection interface of the PD while a second load isapplied by the PD to the channel in place of the first load.
 15. Anapparatus as in claim 14 wherein: the first signal value comprises afirst voltage measurement and a first current measurement; the secondsignal value comprises a second voltage measurement and a second currentmeasurement; and the controller is configured such that when it performsthe processing operation it is configured to calculate a voltagedifference as the difference between the first and second voltagemeasurements, calculate a current difference as the difference betweenthe second and first current measurements, and divide the voltagedifference by the current difference.
 16. An apparatus as in claim 13wherein the controller is configured such that when it selects it isconfigured to choose a method for calculating resistance based on theavailability of the PSE signal measurement and the PD signal measurementand pick an input parameter associated with the chosen method.
 17. Anapparatus as in claim 16 wherein the controller is configured such thatwhen it selects it is further configured to choose a method that is mostaccurate among a variety of methods.
 18. An apparatus as in claim 17wherein the controller is configured such that when it chooses a method,if a PD signal measurement is available and if the PD is capable ofmeasuring local voltage and current for two different load values, it isfurther configured to: select as the input parameter the PD signalmeasurement.
 19. An apparatus as in claim 18 wherein the controller isconfigured such that when it receives indication that the PSE signalmeasurement or PD signal measurement is available it is configured to:obtain the PD signal measurement from the PD, the PD signal measurementbeing based on (i) a first signal value read at a connection interfaceof the PD while a first load is applied by the PD to the channel and(ii) a second signal value read at the connection interface of the PDwhile a second load is applied by the PD to the channel in place of thefirst load.
 20. An apparatus as in claim 19 wherein: the first signalvalue comprises a first voltage measurement and a first currentmeasurement; the second signal value comprises a second voltagemeasurement and a second current measurement; and the controller isconfigured such that when it performs the processing operation it isconfigured to subtract the second voltage measurement from the firstvoltage measurement and divide the difference by the first currentmeasurement minus the second current measurement.
 21. An apparatus as inclaim 17 wherein the controller is configured such that when it choosesa method, if a PD signal measurement is not available or if the PD isnot capable of measuring local voltage and current for two differentload values, it is further configured to: if the PSE is capable ofmeasuring local voltage and current for two different load values, thenselect as the input parameter the PSE signal measurement; otherwise ifthe PSE is capable of measuring local voltage and current for one loadvalue and the PD is capable of reporting its power load, then select asthe input parameter the PSE signal measurement and the PD signalmeasurement; otherwise select as the input parameter the PSE signalmeasurement.
 22. An apparatus as in claim 21 wherein: if the PD signalmeasurement is selected, the PD signal measurement is based on aplurality of measurements taken at different loads, the plurality ofmeasurements being used to eliminate any offset or gain errors.
 23. Anapparatus as in claim 16 wherein the controller is configured such thatwhen it selects it is further configured to choose a method that isfastest among a variety of methods.
 24. An apparatus as in claim 13wherein the controller is further configured to: if the resistance ishigher than a pre-determined value, instruct the PSE to cease applyingpower to the PD.
 25. An apparatus comprising; memory; a networkcontroller; and means for: receiving indication that a Power SourcingEquipment (PSE) signal measurement is available from a PSE or a PoweredDevice (PD) signal measurement is available from a PD; selecting, as aninput parameter to a processing operation, at least one of the PSEsignal measurement or the PD signal measurement; and performing theprocessing operation to calculate a resistance value indicative of aresistive power loss through the channel between the PSE and PD based onthe input parameter.